My house is very old I have lived here 3 yrs and never had a problem however about 3 days ago I stopped getting hot water in my tub. I have taken off the face plate and there is no cartridge like in any of the videos. There is the copper fitting and then the rest is plastic. Please help

Is this a 2 knob faucet? Do you have a brand name? How often do you flush the minerals out of the water heater?